Lead bullion is usually produced by smelting in a blast furnace and, depending upon the type of ore being treated, it generally contains signficant amounts of impurities, such as copper, iron, zinc and arsenic. In order to produce marketable lead, the molten lead is subjected to a series of refining operations. The aforementioned impurities have traditionally been removed from molten lead by a drossing operation.
The drossing operation comprises cooling molten lead from a temperature of about 800.degree. C. to a temperature above the melting point of lead. As the molten lead cools, the solubility of the elements or compounds thereof (e.g., copper sulfide) decreases as the temperature falls, and the elements or compounds are rejected from the solution. The rejected elements or compounds float to the surface of the lead either as solids or as liquids which are skimmed or otherwise removed from the top portion of the molten lead to provide a refined lead product.
The presence of sulfur reduces the solubility of copper in lead. Thus the amount of copper remaining in the bullion after the treatment described hereinabove, can be further reduced by treatment with sulfur as the seed material.
Drossing has generally been accomplished by pouring large quantities of molten lead into a kettle or pot which allows for the formation of lead oxide fumes and lead oxide dross. A disadvantage of this process is the loss of lead due to oxidation and the production of lead fumes which, if not controlled, contribute to hazardous working conditions. A further disadvantage of this type of drossing operation is the significant labor required to skim the dross from the kettle, as well as exposure of the workforce to potentially toxic conditions, such as to lead poisoning.
The prior art is replete with proposed solutions to the problem of the drossing of lead. For example, in U.S. Pat. No. 3,317,311, a process is provided for the copper drossing of lead bullion whereby molten lead is fed to a vessel in which an upper portion is maintained at a high enough temperature to prevent any accretions from forming on the inside of its walls, and a lower portion which is drastically cooled to form accretion banks consisting of a matrix of solid lead containing precipitated particles of impurities, such as copper, or copper sulfides and arsenides. Relatively cold drossed lead is drawn from the bottom of the vessel, and from time to time the cooling is interrupted to melt off the accretion banks to cause copper-containing particles to float to the top of the molten lead in the vessel and dissolve in a matte layer, the molten matte being thereafter tapped at intervals. The drastic cooling referred to is effected by the direct spraying of water upon the outer surface of the lower portion of the vessel.
As will be apparent, this process is semi-continuous in that the drossing operation must be periodically terminated to remove lead deposits from the sidewalls of the vessel.
Another approach to the problem is disclosed in U.S. Pat. No. 3,260,592. According to the patent, copper is separated from lead bullion by using a process comprising continuously mixing bullion at a relatively high temperature with recirculated bullion having a lower temperature and a lower copper content whereby the mixture has an intermediate temperature to effect the separation of copper dross therefrom. The mixture is cooled and a proportion of it recirculated for admixture with additional hot bullion, the balance of the cooled mixture being discharged from the operation.
The hot bullion in the aforementioned process preferably flows downwardly from the upper zone of the bath and continues to flow downwardly after admixture with the cooler bullion, while the dross particles which separate from the mixture rise to the top and float on the free surface. The surface of the bath is maintained at a temperature above the melting point of the dross, whereby the molten dross is caused to flow through a suitably positioned discharge opening which eliminates the necessity of manually skimming the dross.
By cooling the hot bullion by admixture with the circulated cooler bullion, the dross separates out within the body of the mixture, thereby substantially avoiding the formation of the solid accretions on the walls of the containing furnace, kettle, or the like. The process described by the patent requires the recirculation of large amounts of lead to effect the desired cooling substantially in excess of the amount discharged. A disadvantage of this process is that a large inventory of lead for recirculation is required. In addition, the process is energy intensive.